Various methods and systems are known in the art for tracking the coordinates of objects involved in medical procedures. Some of these systems use magnetic field measurements. For example, U.S. Pat. Nos. 5,391,199 and 5,443,489, whose disclosures are incorporated herein by reference, describe systems in which the coordinates of an intrabody probe are determined using one or more field transducers. Such systems are used for generating location information regarding a medical probe or catheter. A sensor, such as a coil, is placed in the probe and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by magnetic field transducers, such as radiator coils, fixed to an external reference frame in known, mutually-spaced locations.
Additional methods and systems that relate to magnetic position tracking are also described, for example, in PCT Patent Publication WO 96/05768, U.S. Pat. Nos. 6,690,963, 6,239,724, 6,618,612 and 6,332,089, and U.S. Patent Application Publications 2002/0065455 A1, 2003/0120150 A1 and 2004/0068178 A1, whose disclosures are all incorporated herein by reference. These publications describe methods and systems that track the position of intrabody objects such as cardiac catheters, orthopedic implants and medical tools used in different medical procedures.
It is well known in the art that the presence of metallic, paramagnetic or ferromagnetic objects within the magnetic field of a magnetic position tracking system often distorts the system's measurements. The distortion is sometimes caused by eddy currents that are induced in such objects by the system's magnetic field, as well as by other effects.
Various methods and systems have been described in the art for performing position tracking in the presence of such interference. For example, U.S. Pat. No. 6,147,480, whose disclosure is incorporated herein by reference, describes a method in which the signals induced in the tracked object are first detected in the absence of any articles that could cause parasitic signal components. Baseline phases of the signals are determined. When an article that generates parasitic magnetic fields is introduced into the vicinity of the tracked object, the phase shift of the induced signals due to the parasitic components is detected. The measured phase shifts are used to indicate that the position of the object may be inaccurate. The phase shifts are also used for analyzing the signals so as to remove at least a portion of the parasitic signal components.
In some applications, the distortion of the magnetic field is measured and/or compensated for by conducting measurements using several magnetic field frequencies. For example, U.S. Pat. No. 4,829,250, whose disclosure is incorporated herein by reference, describes a magnetic system for determining the relative orientation between a fixed frame of reference and an unconstrained object. Mutual coupling between three orthogonally-disposed transmitting coils driven by a multi-frequency source and three orthogonal receiving coils produce sets of analog voltages. The analog voltages are sampled, digitized and processed using a Fast Fourier Transform (FFT) device to yield directional components for determining the pitch and yaw angles. By using the multi-frequency source to drive the transmitting coils and by deriving coordinate component measurements on at least two discrete frequencies, errors in the results due to eddy currents in surrounding conductive structures can be compensated for.
As another example, U.S. Pat. No. 6,373,240, whose disclosure is incorporated herein by reference, describes a method for tracking an object. The method includes producing an unperturbed energy field at a plurality of predetermined frequencies in the vicinity of the object, and determining a characteristic of a perturbing energy field induced responsively to the unperturbed field, due to the introduction of an article into the vicinity of the object. The method further includes receiving a plurality of resultant signals responsive to the unperturbed and perturbing energy fields generated at a location of the object after introduction of the article, determining an optimal frequency for the unperturbed energy field from amongst the plurality of predetermined frequencies responsive to a parameter of the resultant signals, and determining spatial coordinates of the object responsive to the resultant signal at the optimal frequency.